A little over a year ago I posted a list of knots on the Yahoo Hammockcamping Group I consider to be "Essential Outdoor knots". My Grandfather taught me tons of knots. He was an old time honest-to-goodness cowboy and outdoorsman and he never ceased to amaze me at the number of things one could do with the right rope and a good knot. I can tie about 30 knots from memory, but the Essential Outdoor Knots are those that I find myself using most often and teaching to others who are along for their first trip. Generally they are easy to tie, easy to untie and have multiple uses.
I have over the years added and subtracted various knots, replacing them with similar knots that are either easier to tie or untie, but serve the same purpose. I now call the list "10 Essential Knots for the Outdoorsman". With an addendum to a few other notable knots!
10 Essential Knots for the Outdoorsman
End knots: End knots are tied in the end of a cut rope to keep it from fraying. Whipping, melting or back braiding is considered superior, but when time is of the essence, these knots will do the trick. They may also be used for 'stopper knots'. Used primarily to stop the bitter end of the rope from slipping past a primary knot (for safetys' sake), but also used to stop the rope at a certain point when using a pulley or for measurement.
Double Overhand (Ashley Stopper)
Loops, or knots that form loops, are very numerous. Even the old standby, the Bowline, has many (5?) different variations. Some are better tied into the end of the rope and some are better in the middle (span) of the rope. The bowline and figure eight have variations for both needs. I leave off the 'Figure Eight on a Bight' for a reason. This is one of my favorite knots, cool to tie and it looks good lying along the rope when it is tied. But after it is loaded it can be a real bear to untie. So it gets an honorable mention, but that's it.
Bowline on a Bight* Honorable mention, the bowline on a bight is a great way to make a hande in order to drag something, like a heavy log, or to make shoulder straps for pulling an injured person on a sled. It doubles the area of contact at the loop causing less hand pain and more purchase.
Alpine Butterfly (Linesman's Loop)
Bends, for joining two ropes, or the ends of one to form a loop.
Double Fisherman's Knot* A variant of the Ashley Stopper described above.
Sheet Bend and Double Sheet Bend
Adjustable knots, like the Drivers' (Trucker's) Hitch and the Midshipmans' Hitch are worth their weight in gold.
Trucker's/Driver's Hitch This hitch has many names, but this is the only way to tie it properly. You can also double wrap the bitter end to get a better mechanical advantage. Once wrapped three times or so the rope will generally grab and hold itself while you finish the half hitches, very handy.
Midhipman's Hitch (Taught Line) This hitch also goes by many names, tent-line, tight-line and adjustable hitch ar just a few. It is actually a rolling hitch variant. The same hitch tied to a spar is a Rolling Hitch. Adding wraps under the half hitch will help it grab slick rope better, adding wraps above the half hitch will help it grap slick spars better.
Prusik Hitch* Honorable mention: Technically a slide and grip hitch, but it has many uses around camp.
Exploding Hitches. I use the Mooring Hitch, Exploding Clove Hitch and the Highwayman all the time. Very quick and easy to tie and untie. While I have used the Exploding Clove Hitch to suspend my hammock, I don't recommend it. The Mooring Hitch can be used to suspend a hammock as it is a sliding (adjustable), locking hitch that is easy to untie (exploding). The Exploding Clove Hitch is less reliable, but good when you need a quick grab on a line that will have steady tension on it. The Highwayman Hitch is great for hanging stuff from the ridgeline, like stuff sacks, or anything else you can add a short piece of cord to.
* Honorable Mention: Slackers. Rope too long? Don't cut it, use these handy hitches to take up that slack.
Clove Hitch* Honorable mention: Excellent for holding the tarp side up when used in conjunction with a trekking pole or stick. Easy to adjust up or down as well, just pull a little slack and slide the whole thing. I have also used it to pitch my hammock with just the trekking poles although I used extra guy outs to stabilize the poles.